1. Field of the Invention
The present invention relates to a cylindrical outer surface scanning apparatus and a method for use therewith. More particularly, the present invention relates to: a cylindrical outer surface scanning apparatus comprising a recording drum having a cylindrical outer surface for mounting a plate thereon, and an exposure section for scanning the drum with a light beam in a circumferential direction as well as an axial direction of the drum to perform an exposure process for the plate; and a method for use with the cylindrical outer surface scanning apparatus.
2. Related Art Statement
Conventionally, color printed materials are produced through a number of processes such as an exposure process (which serves as an image recording process), a printing process, and the like. Prior to the exposure process, an original image of a color printed material is separated into a plurality of colors, which typically are: Y(Yellow), M(Magenta), C(Cyan), and K(Kuro, i.e., “black”). Thus, image data of the respective colors are generated. Such image data are supplied to a cylindrical outer surface scanning apparatus which is used for an exposure process. The cylindrical outer surface scanning apparatus incorporates a recording drum. On the outer surface of the recording drum, a sheet-shaped plate which serves as an image recording material, e.g., a so-called PS plate(Presensitized Plate), is mounted. A “PS plate” is a plate comprising a plate material (e.g., an aluminum plate, a plastic sheet, or paper) and a photo-sensitive layer preapplied on the plate material.
In the exposure process, the cylindrical outer surface scanning apparatus subjects the plate which is mounted on the outer surface of the recording drum to an exposure process in order to form an image of each of the respective separated colors on a plate based on the image data thus supplied. In other words, in the case where the original image is color-separated into Y, M, C, and K, the cylindrical outer surface scanning apparatus draws images of the four different colors on four plates.
A printing machine which is used in a printing process applies inks to the plates which have been exposed, each ink being in a color corresponding to the associated plate, so as to overlay the respective images on a final color printed material. If the images of one or more colors are misaligned with each other when overlaid, the result color printed material will be of an inferior quality. In order to prevent such misalignment between images, positioning holes for the printing process, which are used as a positioning reference during the printing process, are formed in predetermined positions in the plate, prior to the exposure process. Each plate can be positioned in place by fitting pins which are provided on a printing drum of the printing machine into the positioning holes for the printing process. In some cases, e.g., where the specific printing machine to be used is still undecided at the prepress stage, the positioning holes for the printing process may be formed after the prepress.
Misalignments between images may also occur if the positions of one or more images drawn during the exposure process are misaligned with respect to the plates of the corresponding colors. In order to prevent such image misalignments with respect to the plates during the exposure process, positioning pins for positioning each plate in place are provided on the outer surface of the recording drum of the cylindrical outer surface scanning apparatus. Together with the holes for the printing process, positioning notches for the recording drum are provided along one end of the plate, the positioning notches being configured so as to receive the positioning pins. Thus, the positioning notches for the recording drum and the positioning holes for the printing process are formed in each plate prior to an exposure process. During the exposure process, the positioning notches for the recording drum can be used as a reference for aligning the images to be exposed. During the printing process, the positioning holes for the printing process can be used as a reference for aligning the images to be printed.
Each plate is mounted on the outer surface of the recording drum of the cylindrical outer surface scanning apparatus. The plate is subjected to an exposure process while rotating the recording drum at a high speed (e.g., 1000 rpm). In order to prevent the plate from dropping off the outer surface of the recording drum, the plate is pinched by means of clamps at the leading end and the trailing end, and held in close contact on the recording drum surface with a negative pressure applied from the recording drum surface. Since it is highly dangerous if the plate drops off during an exposure process, latch holes are formed in the plate as safety means. Through the latch holes, the plate is latched on latch pins which are provided on the recording drum surface, whereby the plate is prevented from dropping off.
FIG. 26 is a schematic diagram showing the relative positions of holes which are provided in a plate with various purposes and pins which are provided on the recording drum. For conciseness, the holes for the printing process, which are to be provided in accordance with the specifications of the printing machine used, are omitted from FIG. 26. In FIG. 26, positioning pins 170a and 170b and latch pins 175a and 175b are provided on the outer surface of the recording drum 160. In the plate 150, positioning notches 151a and 151b for the recording drum are formed in positions corresponding to the positioning pins 170. The positioning notch 151a have a semicircular shape having the same inner diameter as that of the positioning pin 170a. The positioning notch 151b has an elongated semicircular shape as if the semicircular shape of the positioning pin 170b were elongated in the lateral direction in the figure. An intra-central intra-central pitch between the positioning pins 170 and an intra-central pitch between the positioning notches 151 are both set equal to a pitch J. Also in the plate 150, latch holes 152a and 152b are formed so as to correspond to the positions of the latch pins 175a and 175b when the positioning pins 170 are fitted in the positioning notches 151. Each of the latch holes 152a and 152b is formed so as to be larger than the latch pins 175 in both vertical and lateral directions in the figure.
When the plate 150 is mounted on the recording drum 160, the plate 150 is transported by a transporter (not shown) in an X direction shown in FIG. 26 until positioned in place as the positioning pins 170 fit into the positioning notches 151. More specifically, the plate 150 is positioned along the axial direction of the recording drum 160 as the positioning pin 170a fits into the positioning notch 151a, and along the circumferential direction (i.e., the X direction) as the two positioning pins 170 fit into the two positioning notches 151. Once the plate 150 is positioned, the plate is latched on the latch pins 175 through the latch holes 152. Since the latch holes 152 are formed larger than the latch pins 175, some interspace is left between the latch holes 152 and the latch pins 175 in a latched state.
FIG. 27A is a schematic diagram illustrating leading-end clamps 180a, 180b, and 180c provided on the recording drum 160. FIG. 27B is a schematic diagram illustrating the plate 150 being stabilized on the recording drum 160. For conciseness, the aforementioned latch pins 175 and the latch holes 152 are omitted from FIG. 27A. As shown in FIG. 27B, the plate 150 is stabilized as the leading-end clamps 180a to 180c pinch the plate 150 against the recording drum 160 at one end thereof.
However, the aforementioned method of positioning and stabilizing the plate 150 onto the recording drum 160 has first to third problems described below.
(First Problem)
A minimum lateral width Lmin of a plate 150 which can be positioned on the recording drum 160 (shown in FIG. 26) is equal to a pitch J of the positioning pins 170 plus a margin t on either side thereof, i.e., Lmin=J+2t. On the other hand, a maximum lateral width Lmax of a plate 150 which can be exposed is limited by a dimension of the recording drum 160 along the axial direction.
However, since there is a desire to support various plate geometries, it may become necessary to perform an exposure process for a plate 150 having a pitch J which is smaller than the minimum lateral width Lmin. In order to address such situations, the pitch J may be simply reduced so as to correspond to shorter plates 150. However, a reduced pitch J will generally result in a poorer positioning accuracy when positioning a plate 150 having a lateral width which is closer to the maximum lateral width Lmax. Another method for supporting a minimum lateral width Lmin shorter than the pitch J might be to provide a further positioning pin between the two positioning pins 170, so that three (or more) positioning pins will be used to position the plate 150 in place.
According to the latter method, a plate 150 having a lateral width greater than the pitch J can be positioned by fitting the three or more positioning pins into three or more corresponding positioning notches. Under such a positioning method, however, minute discrepancies in the dimensional accuracy of the positions of the positioning pins and the positions/shapes of the positioning notch may affect the positioning accuracy of the plate 150 more substantially than in the case of the method of using two positioning pins for positioning, thereby resulting in a poorer stability of positioning accuracy. Specifically, it may often be the case that two of the three (or more) positioning pins are fitted in the corresponding positioning notches while the other positioning pin(s) is not in proper contact with the corresponding positioning notch(s).
(Second Problem)
As shown in FIGS. 28A and 28B, after positioning notches 151 are formed in the plate 150, the plate 150 is transported onto the recording drum 160. In general, this transportation of the plate 150 is achieved by means of transportation rollers or the like (see the transition from a state shown in FIG. 28A to a state shown in FIG. 28B). Depending on the transportation positioning accuracy of the transportation rollers and/or the flexure of the plate 150, the positioning pins 170 may not fit into the positioning notches 151. FIG. 28C illustrates an exemplary case where a plate 150 which has been transported onto the recording drum 160 by means of the transportation rollers the positioning pin 170b first comes in contact with the positioning notch 151b. In this case, the plate 150 rotates while sliding along a T direction in FIG. 28(C) within the bounds of the positioning notch 151b. Since the plate 150 in this case will eventually rotate around a center of rotation which exists at a position along the edge of the positioning notch 151b tending toward the center of the plate 15, the positioning pin 170a may not fit in the positioning notch 151a, so that the plate 150 is not properly positioned on the recording drum 160.
(Third Problem)
FIGS. 29A and 29B are longitudinal cross-sectional views illustrating how the plate 150 is positioned and stabilized on the recording drum 160, as seen from an s direction in FIG. 27B. The longitudinal cross-sectional view of FIG. 29A illustrates the neighborhood of one of the positioning pins 170 and a corresponding one of the leading-end clamps 180. The longitudinal cross-sectional view of FIG. 29B provides an enlarged view of the neighborhood of the positioning pin 170. Referring to FIG. 29A, the positioning pin 170 comprises a cylindrical-shaped pin 171 (having a radius r) being internally fastened with a bolt 172, thereby being fixed on the recording drum 160. The leading-end clamp 180 is disposed so as to pivot around a leading-end clamp axis 181. At one end, the leading-end clamp 180 is biased by a spring 182 so as to pivot in an Fcl direction in FIGS. 29A and 29B and clamp one end (i.e., “the leading end”) the plate 150. The plate 150 is transported onto the recording drum 160 by means of the transportation rollers 190 of the transporter, at a transportation angle (angle M) with respect to a normal of the central axis of the positioning pin 170. Then, the plate 150 is transported by means of the transportation rollers 190 in an Fr direction shown in FIG. 29B, until the positioning pin 170 fits in the positioning notch 151 formed in the plate 150 (as represented by the plate 150a). Next, the leading-end clamp 180 is pivoted in the Fcl direction, so that the leading end of the plate 150 is pinched down and stabilized on the recording drum 160. Thereafter, the recording drum 160 is rotated in an Fd direction, so that the plate 150 is completely taken off the transporter and wound around the outer surface of the recording drum 160 in close contact thereto (as represented by the plate 150b).
Now, the manner in which reference notch 151 receiving the positioning pin 170 is clamped will be described in detail. As shown in FIGS. 29A and 29B, when the plate 150 is transported onto the recording drum 160 by the transporter so that the positioning pin 170 is fitted in the reference notch 151, the reference notch is at a distance H off the outer surface of the cylindrical recording drum 160, where the distance H is given as follows:H≧r·tan M.
Next, the leading end of the plate 150 is clamped by the leading-end clamp 180 while receiving a driving force in the Fr direction from the transportation rollers 190. Through this clamping operation, the peripheries of the reference notches are pressed against the outer surface of the cylindrical recording drum 160. In other words, the peripheries of the reference notches must travel the distance H while being in contact with the positioning pins 170 under the driving force applied in the Fr direction. Since the plate 150 is prevented from moving in a direction perpendicular to the outer surface of the cylindrical recording drum 160 due to a friction force against the positioning pins 170, deformation occurs around the reference notches, such that the deformed portions do not come in close contact with the outer surface of the cylindrical recording drum 160. FIG. 30 is a plan view schematically showing the plate 150 which has been deformed in the aforementioned manner. In FIG. 30, the deformed portions Ua and Ub, which are not in close contact with the outer surface of the cylindrical recording drum 160, are lifted off the recording drum 160 relative to the regions of the plate 150 which are in contact with the recording drum 160. If the deformed portions Ua and Ub are in an exposure region W of the plate 150 and the amount of lift exceeds the depth of focus of an exposure head optical system, a blur will occur in the image which is exposed deformed in the portions Ua and Ub. When such a blur occurs in the exposed image, the plate 150 cannot be used for printing because it will produce an unsatisfactory printing result.
In order to reduce the aforementioned distance H, it may be conceivable to transport the plate 150 in a direction perpendicular to the central axis of the positioning pins 170 (i.e., angle M=0 in FIG. 29A). However, it will be difficult to align such a recording drum 160 with a transporter. Coupled with an influence of the flexure of the plate 150, etc., it is difficult to reduce the distance H to zero, and hence deformation of the plate 150 cannot be prevented.